Method and resin solution for splicing carbonized polyacrylonitrile material

ABSTRACT

A method is provided for splicing carbonized PAN tows in which the tows to be spliced are cut square and coated at the ends thereof with a resin solution, following which the coated ends are joined together, heated so as to cure the resin solution and then pinched to compact the resulting splice. The resin solution is comprised of a mixture of a solid acrylic polymer, a liquid acrylic polymer, a solid epoxy resin and a solvent.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and materials for splicing orotherwise joining together carbonaceous materials, and more particularlyto methods and apparatus for splicing carbonized polyacrylonitrile tows.

2. History of the Prior Art

During the manufacture of carbon fibers from polyacrylonitrile (PAN)precursor material, it is frequently necessary or desirable that tows ofthe polyacrylonitrile precursor material which have been oxidized andthen carbonized be spliced or otherwise joined together. Splicing may benecessary, for example, where a plurality of the carbonized tows aresimultaneously handled such as by winding on creels or bobbins. If oneor more of the tows are shorter in length than the others, then splicingof the shorter lengths of tow to other lengths of tow becomes necessaryso as not to interrupt the simultaneous winding or other processing ofthe tows. Splicing of carbonized tows may also be necessary where acontinuous length of tow of specified length or poundage is to beprovided. In such instances the processed length of tow may be too shortso that one or more additional lengths of tow must be spliced thereto.

Carbonized PAN tows have a relatively hard composition and are oflimited flexibility. Consequently, it is very difficult to join togethertwo lengths of such tows by knotting or tying. However, even if knottingor tying were otherwise feasible, the equipment used to handle andprocess the tows typically advances the tows through eyelets and aroundsharp turns so as to prevent the use of tying or knotting.

In some instances carbonized PAN tows have been spliced together usingan adhesive such as dissolved plexiglass. Such adhesives have been foundto provide a splice which is relatively bulky and which in any event istoo stiff to permit the tows and included splice to easily negotiate thesharp turns and eyelets within the processing equipment.

A satisfactory splice of two tows of carbonized PAN must be relativelystrong and at the same time sufficiently flexible and compact so thatthe splice may readily negotiate the eyelets, sharp turns and otherobstacles present in the equipment used to process such tows. Theseobjects must be accomplished in the face of the carbonized tows whichare themselves relatively stiff and inflexible and have relatively hardand smooth surfaces so as to resist adhesive penetration and bonding.

BRIEF DESCRIPTION OF THE INVENTION

These and other objects are accomplished in accordance with theinvention by an improved method of splicing which is preferably utilizedin conjunction with an improved splicing adhesive in the form of a resinsolution. In methods of splicing according to the invention the ends ofa pair of lengths of carbonized PAN tow to be spliced are first cutsquare and are then coated with a resin solution such as by dipping theends in the solution. The coated ends are then joined together, andwhile in this position are heated at a sufficient temperature and for asufficient time to evaporate solvent within the resin solution and curethe resin solution. The resulting splice is then pinched together so asto smooth and compact the splice.

A preferred form of resin solution in accordance with the invention iscomprised of an acrylic polymer, an epoxy resin and a solvent. In oneexample thereof, the acrylic polymer is made up of a solid acrylicpolymer having a high molecular weight on the order of about 260,000 anda liquid acrylic polymer having a much lower molecular weight on theorder of about 7000. The solid epoxy resin comprises the tetraglycidylether of tetraphenylethane, and the solvent comprises methylenechloride. In such example, the resin solution is comprised ofapproximately 47.8 parts by weight of the solid acrylic polymer,approximately 37.2 parts by weight of the liquid acrylic polymer,approximately 15.0 parts by weight of the solid epoxy resin andapproximately 900 parts by weight of the solvent.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings, in which:

FIG. 1 is a block diagram of the successive steps in a preferred methodof splicing carbonized polyacrylonitrile material in accordance with theinvention;

FIG. 2 is a perspective view of the ends of two tows of carbonizedpolyacrylonitrile material which have been cut and coated with a resinsolution in accordance with the method of FIG. 1;

FIG. 3 is a perspective view of a heating mandrel showing the manner inwhich the coated tow ends of FIG. 2 are brought together and heated tocure the resin and form the splice; and

FIG. 4 is a perspective view of the completed splice after the tow endshave been pinched together for compacting in accordance with the methodof FIG. 1.

DETAILED DESCRIPTION

FIG. 1 depicts a preferred method of splicing together carbonizedpolyacrylonitrile (PAN) material in accordance with the invention. Themethod of FIG. 1 is described in connection with the splicing of twotows of carbonized PAN.

In a first step 10 shown in FIG. 1 the ends of two lengths of tow to bespliced together are cut square. This is accomplished by cutting eachtow end through a plane which is generally normal to the central axis ofthe two to insure a relatively square end in preparation for splicing.

In a second step 12 illustrated in FIG. 1 the end of each of the twotows to be spliced together is coated with a resin solution. This may beaccomplished by dipping each of the tow ends into the resin solution adesired distance such as 3/4".

FIG. 2 depicts two different lengths 14 and 16 of carbonized PAN tow.The lengths 14 and 16 have ends 18 and 20 respectively which have beencut square pursuant to the first step 10 of FIG. 2. The ends 18 and 20have coatings 22 and 24 respectively of the resin solution as a resultof the second step 12 of FIG. 1.

In a third step 26 of FIG. 1 the coated portions of the tow lengths 14and 16 are brought together. This means that the tow length 14 isdisposed relative to the tow length 16 such that the lengths 14 and 16overlap with the coatings 22 and 24 being generally coextensive.

The step 26 and a following step 28 in the method of FIG. 1 can beaccomplished in conjunction with a heating mandrel 30 shown in FIG. 3.The heating mandrel 30 has a metal plate 32 mounted on the top thereofand containing a slot 34 therein across the width thereof. The twolengths 14 and 16 are disposed such that the coated portions of the ends18 and 20 thereof are overlapped within the slot 34 so as to accomplishthe step 26 in the method of FIG. 1. In the next step 28 the overlappingtow ends 18 and 20 are heated to cure the resin and form a splice. Theheating mandrel 30 contains heating coils (not shown) which areenergized to provide a controlled amount of heat to the plate 32 at thetop of the mandrel 30 and the included tow ends 18 and 20. In thepresent example the tow lengths 14 and 16 are held by hand in thepositions shown in FIG. 3 for 30-60 seconds in the presence of atemperature within the slot 34 of approximately 450° F. This temperatureand residence time range have proven adequate to substantially cure theresin and form the splice using a resin solution as described hereafter.

In a fifth and final step 36 of FIG. 1 the formed splice is removed fromthe slot 34 within the plate 32 at the top of the heating mandrel 30 andthe joined ends 18 and 20 of the tow lengths 14 and 16 are pinchedtogether either by hand or using an appropriate instrument so as tocompact the splice.

The resulting splice which is shown in FIG. 4 has been found to haveadequate strength and flexibility for further processing of the towlengths 14 and 16. At the same time the splice itself is relativelycompact and is relatively freely drawn through eyelets and overrelatively sharp edges in the processing equipment.

In accordance with the invention the resin solution used in the methodof FIG. 1 is preferably comprised of an acrylic polymer, an epoxy resinand a solvent. Such mixture when made in the right proportions resultsin splices of adequate strength and flexibility despite the limitedflexibility of carbonized PAN tows and the difficulty in penetrating andbonding to the relatively hard material that characterizes such tows. Atthe same time the acrylic polymer cross-links with the epoxy resin toprovide the resulting splice with high solvent resistance. High solventresistance is important where the carbonized PAN tows and the includedsplice therebetween are to be subsequently treated with materialcontaining a solvent. For example where the carbonized PAN tows areprepregged by being coated with a resin mixture in preparation forfurther processing, such resin mixture contains a solvent which mightotherwise attack the bonding adhesive within the splice if such adhesivewere not solvent resistant.

As shown by the examples described hereafter, the nature and quantity ofthe acrylic polymer can determine not only the strength but also theflexibility of the splice. The epoxy resin also plays an important rolein determining the strength and flexibility of the splice. The acrylicpolymer and the epoxy resin cross-link to provide high solventresistance.

The acrylic polymer in the present example is comprised of a mixture ofa first acrylic polymer having a relatively high molecular weight andbeing solid in form and a second acrylic polymer having a considerablylower molecular weight and being in the form of a liquid. Variations inthe relative proportions of the first and second acrylic polymers havebeen found to provide variations in both the strength and theflexibility of the splices. In the present example the first or solidacrylic polymer is comprised of Carboset 525 which is manufactured by B.F. Goodrich Chemical Company and which has a molecular weight of260,000. The second or liquid acrylic polymer comprises Carboset 515which has a molecular weight of 7000 and which is manufactured by B. F.Goodrich Chemical Company. The epoxy resin in the present examplecomprises Epon 1031 which is a solid epoxy resin manufactured by ShellChemical Company. It is the tetraglycidyl ether of tetraphenylethane.The solvent used in the present example is methylene chloride.

In a first experiment 40 grams of Carboset 525 and 10 grams of Epon 1031were dissolved in 450 grams of methylene chloride to form the resinsolution. The resulting solution was used to splice together two lengthsof 12,000 filament Hi-Tex tows manufactured by Hitco, the assigneeherein using the method of FIG. 1. The resulting splice proved to berelatively strong but was determined to be too stiff.

In a second experiment 30 grams of Carboset 525, 10 grams of Carboset515 and 10 grams of Epon 1031 were dissolved in 450 grams of methylenechloride. The resulting resin solution was again used to splice togethertwo different lengths of Hi-Tex tows using the method of FIG. 1. Theresulting splice was strong enough, and proved to be more flexible thanin the first experiment.

In a third experiment 25 grams of Carboset 525, 15 grams of Carboset 515and 10 grams of Epon 1031 were dissolved in 450 grams of methylenechloride. Again the resulting resin solution was used to splice Hi-Textows using the method of FIG. 1. The resulting splice was strong enoughand proved to be even more flexible than in the second experiment.

A number of additional experiments were performed in an effort todetermine an optimum resin solution. The resin solution which proved toprovide the best results was provided by mixing 23.9 grams of Carboset525, 18.6 grams of Carboset 515 and 7.5 grams of Epon 1031 in 450 gramsof methylene chloride.

A further experiment was performed in which the Epon 1031 solid epoxyresin was omitted. In that experiment 28.1 grams of Carboset 525 and21.9 grams of Carboset 515 were dissolved in 450 grams of methylenechloride. When the resin solution was applied to Hi-Tex tows inaccordance with the method of FIG. 1, the resulting splice was found tohave marginal strength.

Ten different splices were made and tested using the preferred resinsolution consisting of 23.9 grams of Carboset 525, 18.6 grams ofCarboset 515, 7.5 grams of Epon 1031 and 450 grams of methylene chloridein conjunction with the method of FIG. 1. The splices were formed using12,000 filament Hi-Tex tows manufactured by the assignee Hitco. The tensplices were tested and determined to have breaking strengths of 20-30pounds. A 10 pound breaking strength is considered adequate for normalprocessing of the carbonized PAN tows. The splices had a suitable degreeof flexibility as well as solvent resistance when used in a prepreggingoperation.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A solution for splicing together lengths ofcarbonized polyacrylonitrile material consisting of an acrylic polymer,an epoxy resin and a solvent, the acrylic polymer consisting of a solidacrylic polymer having a first molecular weight and a liquid acrylicpolymer having a second molecular weight substantially less than thefirst molecular weight, the solution forming a relatively strong andflexible adhesive bond upon curing by heating.
 2. The invention setforth in claim 1, wherein the epoxy resin consists of a solid epoxyresin.
 3. The invention set forth in claim 2, wherein the solid acrylicpolymer has a first molecular weight of about 260,000, the liquidacrylic polymer has a second molecular weight of about 7,000, the solidepoxy resin comprises the tetraglycidyl ether of tetraphenylethane, andthe solvent comprises methylene chloride.
 4. The invention set forth inclaim 2, wherein the solution consists of approximately 47.8 parts byweight of solid acrylic polymer, approximately 37.2 parts by weight ofliquid acrylic polymer, approximately 15.0 parts by weight of solidepoxy resin and approximately 900 parts by weight of solvent.